1. Field of the Invention
The present invention relates to a variable-nozzle type turbo charger equipped with a plurality of nozzle vanes of which the opening degree can be varied, and is capable of varying areas of turbine nozzles at the time when the exhaust gases of an engine are guided from the turbine nozzles formed among the nozzle vanes to a turbine rotor.
2. Description of the Related Art
Internal combustion engines for automobiles can be classified into normal aspiration type engines and supercharged type engines. In recent years, however, the ratio of the supercharged type engines is increasing. The supercharger mechanisms that have been put into practical use include the one of the type driven by an exhaust gas turbine which is generally called a turbo charger and the one of the type mechanically driven which is generally called a supercharger. In the turbo charger, the turbine is rotated by the energy of exhaust gases, and the intaken air is compressed by a compressor directly coupled to the turbine and is supplied to the engine. The turbo-charged engine is usually provided with a waste gate valve for by-passing part of the exhaust gases flowing into the turbine in order to prevent the supercharging pressure from excessively increasing.
When the engine is rotating at a low speed, the flow rate of the exhaust gases is so small that the supercharging by the turbo charger is not sufficient. To cope with this problem, there has been developed a turbo charger in which the areas of the turbine nozzles are decreased to increase the kinetic energy given to the turbine rotor. A variable-nozzle type turbo charger is just one of this type, in which provision is made of a plurality of nozzle vanes of which the opening degree can be changed, in order to vary the areas of the turbine nozzles at the time when exhaust gases of the engine are guided from the turbine nozzles formed among the nozzle vanes to the turbine rotor.
In this variable-nozzle type turbo charger, the opening degree of the nozzle vanes is controlled by using an actuator, and a variety of control methods have heretofore been proposed. For example, Japanese Unexamined Utility Model (Kokai) No. 62-137340 discloses technology according to which the opening degree of the nozzle vanes is minimized during the cold and light-load operation to minimize the areas of the turbine nozzles, in order to increase the pumping work of the engine and, hence, to quickly warm up the engine.
The above-mentioned technology is to quickly warm up the engine without, however, giving a thorough consideration from the start of cranking of the engine to the complete combustion, and there remains a likelihood that a defect may occur as mentioned below. That is, according to the above-mentioned prior art, the actuator is so controlled that the opening degree of the nozzle vanes becomes a minimum, i.e., the areas of the turbine nozzles become a minimum at the time of cold starting. At the start, the flow rate of exhaust gases is smaller than that of after the engine has been started and, hence, the pressure of the exhaust gases (back pressure of the engine) is low. Therefore, when the actuator is so controlled as to minimize the opening degree of the nozzle vanes at the start, the areas of the turbine nozzles become smaller than those of when the actuator is controlled after the engine has been started. As a result, exhaust gases are less exhausted, and the air is not introduced into the combustion chamber to a sufficient degree; i.e., an extremely rich state is established, complete combustion is not accomplished, and the engine is started with difficulty. The above-mentioned technology, therefore, is designed to be effected during the cold and light-load operation condition after the start of the engine causing, however, the back pressure of the engine to be excessively elevated at the time of start, i.e., at the time of cranking, which is not desirable.